Low alternating current (ac) loss superconducting cable

ABSTRACT

The present invention relates to low alternating current “AC” loss high temperature superconducting cables ( 200 ), to methods of fabricating such superconducting cables ( 200 ) and to three phase cables which utilize three individual high temperature superconductor “HTS” tape cables ( 200 ).

FIELD OF THE INVENTION BACKGROUND OF THE INVENTION

[0001] Electrical conductors, such as copper wires, form the basicbuilding block of the world's electric power system, for both powertransmission and distribution. The discovery of high-temperaturesuperconducting compounds in 1986 has led to the development of theiruse in the power industry. This is the most fundamental advancement inconductor technology used for power systems in more than a century.

[0002] Over the past three decades, electric power use has risen about25% -40% in the United States. With this rising demand for power comesan increased requirement for low-cost power. Because of the lack of DCresistance and the low AC losses of superconductors at operatingtemperatures, superconducting devices are being developed forapplication throughout the electric power industry.

[0003] The power industry's future use of superconductors depends on theoverall cost and performance (low power loss) benefits that thesuperconductor wires offer. HTS tape technologies drive down the costs,increase the current-carrying capacity, and improve the reliability ofthe wiring system, thus impacting electric power systems in a variety ofways. These ways include the possibility of greatly reduced size andweight of the wires used in devices such as cables, transformers,motors, and generators. Superconductor wires have many applicationsbecause of their efficiency for carrying electricity and their abilityto carry much higher electrical currents than other conducting materialsin less volume.

[0004] There exists the unmet technical challenge in the power industryof fabricating HTS cables and devices in such a way that they operatewith negligible alternating current (AC) losses. These superconductorscan carry direct current (DC) with negligible losses, but DC is rarelyused in the power industry. AC is the dominant form in most of theworld's power cable-based devices. AC applications of HTS tapes operatewith non-negligible energy losses, the energy escaping in the form ofheat. This impacts the efficiency of the system beyond the mere energyloss since the heat generated must be removed from the environment ofthe device.

[0005] Superconductors operate in the temperature range of 4°-85° K.,far below ambient temperature (298° K.). Thus, superconductors requirerefrigeration, and refrigeration requires continuous expenditure ofenergy. For example, if the heat caused by the electrical currentflowing in superconductor wires is at 77° K. and is dissipated at therate of one watt, then refrigerators must be supplied with approximately10-40 watts of electrical power to dissipate that generated heat. Absentthis refrigeration, the superconductor material would warm itself toabove its superconducting temperature and cease to operate as asuperconductor, thereby eliminating any advantage and, in particular,providing worse performance than conventional copper conductors.

[0006] The heat generated must be eliminated to cost-effectivelymaintain the low temperatures required by the superconductor. Successfulsolution of this problem would reduce operating costs by reducing theadded cooling energy needed.

[0007] The key problem of HTS tapes is that unwanted AC magnetic fieldsare generated by the current flowing in the neighboring HTS tapes, whichcauses AC losses. Because the HTS tape material and geometry isanisotropic, magnetic fields passing perpendicular to the preferreddirection generate significantly greater losses than those of parallelfields. In the present invention, there are no perpendicular magneticfields except for the very ends of the wiring structures, wheredifferent loss mechanisms apply. A discussion of AC losses caused bymagnetic fields can be found in W. T. Norris, J. Phys. D 3 (1970)489-507, or Superconducting Magnets by Martin N. Wilson, OxfordUniversity Press, Oxford, UK 1983.

[0008] It would be highly beneficial to develop a superconductorconfiguration that reduces AC losses and associated very highrefrigeration costs. Practical devices for AC applications could then bewound using wide flat superconductors, the most prevalent and desirableform of high temperature superconductors (HTS).

[0009] Thus, it is an object of this invention to provide a method offabricating superconductor cables such that AC losses due to thepresence of a localized perpendicular component of the self-field iseliminated or minimized.

[0010] It is another object of this invention to provide superconductingcables with minimized AC losses due to the presence of a localizedself-field perpendicular field component.

[0011] It is yet another object of this invention to provide a threephase cable consisting of three superconducting cables with minimizedself-generated AC losses.

[0012] It is yet another object to reduce refrigeration requirementsassociated with the operation of a HTS tapes used in wiring cable-baseddevices by reducing the heat generated by perpendicular magnetic fieldsimpinging on neighboring HTS tapes.

[0013] It is yet another object of this invention to use conventionalHTS wiring tapes and conventional wiring methods in a new wiringconfiguration to create a low cost superconducting device.

BRIEF SUMMARY OF THE PRESENT INVENTION

[0014] HTS tapes may be wound around cable structures in various waysdescribed as “winding configurations”. Winding configurations can bechanged in a variety of ways by changing (1) the size of thesuperconductor wires (width, thickness, shape) on the cable structure,(2) the type of superconductor material used, and (3) the way the tapeis wound on a cable structure itself (spacing to its neighboring wire).

[0015] Surprisingly, it has been determined that eliminating the gapsnormally present when superconductor tapes are wound into cablesprevents significant energy losses and limits the need for cooling ofthe superconductor. The present invention obtains low AC loss results byproviding novel techniques of winding the tape on a cable structure.

[0016] In most applications, the HTS tape is continuously in thepresence of an AC field. The present invention is directed toward HTStape-winding configurations used in applications where the AC frequencyis typically in the range of 50-60 Hz (normal operating frequency in thepower industry). By using HTS tapes instead of standard copper wires,better performance (lower power losses) and lower cost are achieved.However, HTS tapes require cooling, which uses power. The presentinvention is directed to HTS tape wiring configurations designed toachieve low AC losses, thereby reducing refrigeration requirements andenabling superconducting wiring structures to achieve their higherperformance at lower cost.

[0017] A significant source of AC loss is the loss caused by themagnetic fields of the neighboring HTS tapes, said field being generatedby AC current traveling through HTS tapes, in particular, the magneticself-fields that are allowed to form because of gaps between the HTStapes.

[0018] It has now been discovered the superconductors composed ofconventional materials but wound in specified configurations eliminatecertain energy losses commonly present in HTS applications. Theinvention applies broadly to a superconductor winding configuration thateliminates local perpendicular field components.

[0019] This new HTS tape configuration approximates a single current“sheet”, which produces minimal magnetic fields perpendicular to thecurrent flow, thus significantly reducing AC losses.

[0020] The invention comprises a method of fabricating superconductingcables that minimize the AC losses in the main body of thesuperconducting cable and low AC loss superconducting cables. Thebeneficial results of the invention are obtained by fabricatingsuperconducting cables such that superconductors overlap one another sothat gaps between the superconductors are covered by anothersuperconductor.

[0021] Because there are no uncovered gaps, the individual turns of theHTS tapes approximate a single long sheet of current, forcing themagnetic field to be primarily parallel to the surface of the core andsurface of the superconductor. This is a preferential orientationbecause it minimizes or eliminates the component of the magnetic fieldperpendicular to the surface of the superconductor. With no substantialperpendicular field component, the high perpendicular field losses inthe superconductor are eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a magnified view of a typical prior art deviceillustrating the general effects of self-induced magnetic fields on anHTS tape.

[0023]FIG. 2 is a sectional view illustrating a staggered windingconfiguration in a HTS tape wired cable assembly of the presentinvention.

[0024]FIG. 3 is a magnified view illustrating a staggered windingconfiguration in a HTS tape wired cable assembly of the presentinvention.

[0025]FIG. 4 is a sectional view illustrating a lapped windingconfiguration in a HTS tape wired cable assembly of the presentinvention.

[0026]FIG. 5 is a magnified view illustrating a lapped windingconfiguration in a HTS tape wired cable assembly of the presentinvention.

DETAILED DESCRIPTION OF THIS INVENTION

[0027] The present invention relates to superconductor tapes,fabrication methods and configurations that are designed to minimize theAC losses in a superconducting cable or assembly. Superconducting tapesof various compositions are well known. Suitable high-temperaturesuperconductor tapes are for example Bi-2223 superconducting tapes, andinclude, but are not limited to, those superconductor tapes that areformed from any of the following families of superconductive materials:cuprates (such as YBCO or BSCCO), diborides, or metallicsuperconductors.

[0028] Suitable HTS tapes can be flat and can also be elliptical, orrectangular. HTS tapes are typically from about 0.001 mm to about 10 mmthick and from about 0.5 mm to as wide as convenient for the design ofthe superconducting assembly. The HTS tapes can be either monocore ormultifilament, thin or thick film, powder-in-tube or surface-coated, orany variety of high-temperature superconductors where the final form isflat, elliptical, or rectangular.

[0029] A single layer of HTS tape may be used in the lapped embodimentof the invention; a minimum of two HTS layers are required to achievethe benefits of the invention in other embodiments, but it is possibleto have as many layers as are required by design considerations.

[0030] The HTS tapes are wound on a “core” which is used to support theHTS tapes. The core is a flexible cylinder, hollow or solid. This corestructure can range from 0.25 inch to several inches in diameter and canrange from several feet to several miles in length. HTS tapes arepreferably wound at a pitch angle of between about 5° and 85°,preferably at a pitch angle of between about 10° and 30°, relative tothe longitudinal axis of the total core structure to create a cable andto maximize its effectiveness electrically and physically. HTS tapes canalso be wound at different angles relative to the longitudinal axis ofthe core structure to create a cable with different electrical andphysical requirements. The tapes are wound on the core usingconventional fabrication techniques. Any conventional core can beutilized in the process; upon completion of tape wrapping the core mayremain or may be removed.

[0031] These tapes are configured so that they overlap one another suchthat all gaps between HTS tapes are covered by another HTS tape. The HTStapes are essentially parallel conductors terminated together at theends of the superconducting cable.

[0032]FIG. 1 illustrates, in very general terms, how thehigh-temperature superconductor's wires or HTS tapes in the presence ofmagnetic fields create AC losses.

[0033]FIG. 1 shows an example of a general view 100 of prior art HTStapes on a core. The core 114 is used to support the HTS tapes. Acutaway portion of three separate HTS tapes 110A-C is also shown. Thecore 114, shown in FIG. 1, is a small section of a total structure thatHTS tapes 110A-C are wound around.

[0034] The electrical current direction flowing in each HTS tape 110A-Cis shown as 116A-C, respectively. Current 116A flowing in HTS tape 110Ashows the direction of a self-induced magnetic field loop 112A.Self-induced magnetic field loops 112B, and 112C are also shown forcurrents 116B, and 116C, respectively. Self-induced magnetic field loop112A is shown to couple or cross HTS tape 110B. This coupling of fieldlines on HTS tape 110B can range from angles that are perpendicular tothe surface of HTS tape 110B to angles that are parallel to HTS tape110B.

[0035] Also shown in FIG. 1 is a gap 120 between HTS tapes 110A and110B. Note that this gap 120 exists between HTS tapes 110B and 110C aswell, but is not annotated. Because gaps 120 exist, the self-inducedmagnetic fields are able to complete their magnetic loops. Although FIG.1 portrays self-induced magnetic field loops 112A-C as single loops, itshould be noted that there are many magnetic field loops or magneticfield lines emanating from HTS tapes 110A-C in a radial fashion awayfrom each HTS tape 110A-C, where the field strength diminishes as onemoves away from HTS tape 110A-C.

[0036] The coupling of self-induced magnetic field loop 112A on HTS tape110B, when it is nearly perpendicular to HTS tape 110B, creates adeleterious current flow 118 in HTS tape 110B that creates heat loss.Since the electrical currents discussed are AC, this deleterious currentflow 118 is also AC; hence, this loss to heat is called AC loss. Alsoshown is how each HTS tape 110, like HTS tape 110B with current 116B,has its self-induced magnetic field loop 112B, which couples to itsnearest neighbor HTS tape 110C. Thus, this coupling of an HTS tape withits self-induced magnetic field lines (because the magnetic field isallowed to complete its magnetic loop between the HTS tapes) causes AClosses to its neighboring HTS tape.

[0037] Decreasing or eliminating the perpendicular component of themagnetic field that is created by the local self-induced magnetic fieldloop 112A, as shown in FIG. 1, substantially reduces AC losses. Commonwinding techniques allow for winding of an HTS tape of superconductorcables in a manner that causes gaps to form between the superconductorsof the HTS tapes. As current flows through the HTS tapes, these gapsallow perpendicular magnetic fields to form around the HTS tapes, andthese field lines penetrate into adjacent HTS tapes, and thus create AClosses. Above a given magnitude of current, called the “criticalcurrent” flowing in the superconductor, the superconductor will gonormal, that is, no longer be superconducting. For currents at or lessthan the critical current of the superconducting material, the inventiveconfigurations approximate a single-turn current sheet, forcing thecollective fields to be mainly parallel to the surface of thesuperconductor winding, a preferential orientation. Therefore, with nosubstantial perpendicular field component; the high AC losses caused byperpendicular magnetic fields penetrating adjacent HTS tapes areeliminated in the main body of the superconducting assembly.

[0038] When transport currents are at, or less than the critical currentof the superconductor, this approximates a single-turn current sheetwith a constant transport current per unit of axial length along thecable, a situation that substantially minimizes the perpendicular field(with the exception of the end-turn regions). The collective magneticfield, such as loop 212 of FIG. 3, surrounding the approximatedsingle-turn current sheet is almost completely parallel to the surfaceof the HTS tapes in the main body of the windings.

[0039] A first preferred embodiment of the invention is the staggeredwinding embodiment. The staggered winding embodiment of the invention isdescribed more clearly with reference to FIG. 2, which shows a sectionalview of a staggered winding configuration 200 for a first embodiment ofthe present invention. FIG. 2 shows a first HTS tape layer 220Aconsisting of a plurality of HTS tapes 210 that are spirally wound in apath between about 5° and 85°, preferably from about 10° and 30°,relative to the longitudinal axis of the core 212. The application ofthe first HTS tape layer 220A onto the core 212 creates the gaps 120′,as seen most clearly in FIG. 3. A second HTS tape layer 220B is spirallywound in a path identical to that of the first HTS tape layer 220Awhereby the application of the second HTS tape layer 220B effectivelycovers the gaps 120′ created between individual HTS tapes of the firstHTS tape layer 220A.

[0040]FIG. 3 shows a magnified cutaway section of a staggered windingconfiguration 200 for a preferred embodiment of the present invention.FIG. 3 shows HTS tapes 310A-D on the core 212′. HTS tapes 310A-D areseparated by spaces or gaps 120′ (one is shown for demonstrationpurposes). HTS tapes 310A-D are shown on a first HTS tape layer 220A′. Aplurality of HTS tapes 312A-312C of a second HTS tape layer 220B′ areshown arranged on top of the first HTS tape layer 220A′. Each HTS tape312A-D of second HTS tape layer 220B′ overlaps gaps 120′ in the firstHTS tape layer 220A′. For instance, HTS tape 312C covers gap 120′between HTS tape 310C and HTS tape 310D. Current 314A shows thedirection of current flow in HTS tape 310A of first HTS tape layer220A′, whereas a current 316A shows the direction of current flow in HTStape 312A of second HTS tape layer 220B′. All current flows in identicaldirections in all HTS tapes at both first HTS tape layer 220A′ andsecond HTS tape layer 220B′. A self-induced magnetic field loop iscreated by the current flow shown in current flow directions 314A and316A in all HTS tapes 310A-D and all HTS tapes 312A-D, respectively.

[0041] The HTS tapes 110A-D and HTS tapes 210A-C, represented in FIG. 3,depicting an inventive device, are individual high-temperaturesuperconductor tapes. In the figure, HTS tapes 110A-D and 210A-C areshown as flat, but suitable HTS tapes can also be elliptical, orrectangular. In FIG. 3, only two layers are shown, first HTS tape layer330A and second HTS tape layer 330B, but it is possible to have as manylayers as are required by design considerations.

[0042] In a second preferred embodiment, as shown more clearly in FIG.4, a lapped winding configuration 300 is used. Winding an HTS tape suchthat one edge of the HTS tape rests on the surface of a core and theopposite edge rests on an adjacent HTS tape creates the lappedconfiguration.

[0043]FIG. 4 shows a sectional view of a lapped winding configuration300 for the lapped embodiment of the present invention. FIG. 4 shows anHTS tape layer 410 consisting of a plurality of HTS tapes 412 that arespirally wound in a path between 5° and 85° relative to the longitudinalaxis of the core 212′. The application of the HTS tapes 412 onto thecore 212, whereby a first HTS tape underlaps a previous HTS tape andoverlaps a following HTS tape, effectively covers the gaps 120′, asshown in FIG. 3.

[0044] As shown in FIG. 5, a plurality of HTS tapes 412A-I are wound oncore 212′. A current direction 510A and a current direction 510B showthe direction of current in HTS tapes 412A and 412B, respectively. Notshown are all the other current flow lines, which are all in the samedirection as current directions 510A and 510B. Self-induced magneticfield loop caused by the current flow in HTS tapes 412A-I, runs mostlyparallel to HTS tapes 412A-I. In this lapped winding configuration 300there is virtually no perpendicular component to self-induced magneticfield loops for HTS tapes, and therefore no AC losses.

[0045] The winding sections of HTS tapes 412A-I, in the presentembodiment, are winding sections of an individual, high-temperaturesuperconductor tape but could be any number of tapes in parallel. FlatHTS tapes 412A-I are shown, but they may be made elliptical orrectangular.

[0046] HTS tapes 412A-I are wound around core 212′ in a near-parallelpath relative to the longitudinal axis of core 212′. In the presentembodiment, only one HTS tape layer 410 is shown, but it is possible tohave as many layers as are required by design considerations.

1. A low alternating current loss superconducting cable comprising a superconducting tape spirally wound around a longitudinally extending core in a path between about 5° and 85 °relative to the longitudinal axis of the core such that the tape completely covers the longitudinal surface of the core.
 2. The cable of claim 1 wherein the surface is covered by more than one tape.
 3. The cable of claim 1 wherein the superconducting tape is selected from a member of the group consisting of cuprate based, diboride based and metallic superconducting tapes.
 4. The cable of claim 1 wherein the superconducting tape is a tape selected from the group consisting of monocore, multifilament, thin film, thick film, powder-in-tube and surface-coated superconducting tapes.
 5. The cable of claim 1 wherein the superconducting tape is a tape selected from the group consisting of elliptical, and rectangular superconducting tapes.
 6. The cable of claim 1 wherein the superconducting tape has a thickness of from about 0.001 mm to about 10 mm thick.
 7. A low alternating current loss superconducting cable comprising a plurality of superconductor tapes, a portion of such tapes being individually positioned in a first layer around a longitudinal axis and extending longitudinally with such axis, gaps being present between the superconductive material in adjacent tapes of such first layer, and at least one second layer formed of a portion of such plurality of tapes each individually positioned in a second layer around a longitudinal axis and extending longitudinally with such axis, gaps being present between superconductive material in adjacent tapes of the at least one second layer, the superconductive material of the at least one second layer overlapping the gaps in the first layer.
 8. A low alternating current loss superconducting cable as in claim 7, wherein the first layer and at least one second layers are concentric, the at least one second layer having superconductive material that collectively entirely overlaps the gaps in said first layer.
 9. A superconducting cable as in claim 8, wherein the tapes are at least flat in cross section.
 10. A low alternating current loss superconducting cable, comprising a plurality of superconductor tapes, each tape being individually positioned in a first layer around a longitudinal axis and extending longitudinally with said axis, each tape in being located between two immediately adjacent tapes, one lateral edge of each tape underlapping one associated adjacent tape and the other lateral edge of each said tape overlapping the other associated adjacent tape, such that at least a minor portion of the superconductive part of the superconductor tape under or overlaps an associated adjacent tape.
 11. A low alternating current loss superconducting cable as in claim 1, wherein the plurality of superconductor tapes are spirally wound in a path between about 10° and 30° relative to the longitudinal axis of the core.
 12. A low alternating current loss producing superconductor cable comprising superconducting tape spirally wrapped in a gap free lapped configuration on an annular core.
 13. A method of preparing a low alternating current loss superconducting cable comprising wrapping a cylindrical core section with at least one superconductor, such superconductor being positioned around the longitudinal axis of such core such that at least 1% of each winding of such superconductor around such core overlaps an associated adjacent winding.
 14. The method of claim 13 wherein the superconductor overlaps at least 25%.
 15. A method of fabricating low current loss superconducting cables comprising winding a cable of superconducting tape around a core, where the superconducting cable comprises a plurality of superconductor tapes, individually positioning each tape in a first layer around a longitudinal axis and extending longitudinally with said axis, locating each tape between two immediately adjacent tapes, one lateral edge of each tape underlapping one associated adjacent tape and the other lateral edge of each said tape overlapping the other associated adjacent tape, such that at least a minor portion of the superconductive part of the superconductor tape under or overlaps an associated adjacent tape.
 16. A three phase cable consisting of three low alternating current loss superconducting cables, such cable comprising a plurality of superconductor tapes, each tape being individually positioned in a first layer around a longitudinal axis and extending longitudinally with said axis, each tape in being located between two immediately adjacent tapes, one lateral edge of each tape underlapping one associated adjacent tape and the other lateral edge of each said tape overlapping the other associated adjacent tape, such that at least a minor portion of the superconductive part of the superconductor tape under or overlaps an associated adjacent tape.
 18. A three phase cable consisting of three low alternating current loss superconducting cables, such cable comprising a plurality of superconductor tapes, a portion of such tapes being individually positioned in a first layer around a longitudinal axis and extending longitudinally with such axis, gaps being present between the superconductive material in adjacent tapes of such first layer, and at least one second layer formed of a portion of such plurality of tapes each individually positioned in a second layer around a longitudinal axis and extending longitudinally with such axis, gaps being present between superconductive material in adjacent tapes of the at least one second layer, the superconductive material of the at least one second layer overlapping the gaps in the first layer. 